Bell type annealing furnaces have long been used to anneal and spheroidize stacks of coils such as steel strip, wire or rod. The annealing furnace includes an outer cover with a heat source for example gas burners or electrical heating elements that heats the space within outer cover. An inner cover is positioned in the heated space and seals a charge, commonly stacks of metal coil, in a controlled chamber. Ambient initial atmosphere that contains undesirable gases such as oxygen is discharged from the chamber and is continuously replaced with substantially inert furnace gases supplied from an inlet. These furnace gases, commonly referred to as washing gases, for example nitrogen, are ladened with small percentages of hydrocarbons. The gas is used to promote, transfer and circulate heat transferred through the inner cover and provide washing of the stacks of coils as the coils are heated. The hydrocarbons are used to bond to any free oxygen within the furnace in order to prevent the oxygen from attacking the carbon in the annealing metal.
To promote circulation of the furnace gases, a fan is centrally mounted at the base of the furnace. A diffuser plate helps diffuse the furnace gases radially from the central fan radially toward the inner cover of the furnace. Any gases that have been heavily ladened with moisture and other contaminants travel downward and are discharged out through a lower positioned discharge port. Fresh incoming replacement washing gas often referred to as enriching gas is further supplied from the inlet near the bottom of the furnace to be mixed with the previously heated furnace gases. As these gases pass along the inner cover, they are heated up and travel upwards and mix with the previously heated furnace gases.
It has been discovered that the relatively cool enriching gases which have an initial temperature of approximately 100° F. often do not adequately mix and heat up with the previously heated furnace gas which are often at temperatures of approximately 1400° F. If the enriching gases in some cases do not reach a minimum adequate cracking temperature of approximately 900°, concentrated forms of hydrocarbons may be deposited on some coils within the chamber. Furthermore, the cooler gases may prevent some metal coils to reach the desired temperature adequate for proper annealing and spheroidizing. These coils within the annealing furnace may undergo an undesirable phenomenon known as carbon pickup which results in an embrittled metal.
Statistically, most of the above noted problems occur with the coil stacks closest to the enriching gas inlet. The present inlet for the enriching gases is located under the coil stacks. The coil stacks which are round are positioned on the diffuser plate within a round furnace inner cover of substantially larger diameter. If the diffuser plate does not have an upper plate cover, the enriching gases from the inlet can follow an undesirable flow route directly upwardly within the gap that is present between the round inner cover wall and the round coil stacks to prematurely come into contact with the coil stacks located in proximity above the inlet.
What is needed is an improved delivery of furnace enriching gases which will properly be heated and mixed with the previously heated furnace gases to achieve proper mixing, diffusing, and heating before coming into contact with the charge. What is further needed is a diffuser plate which accommodates the improved distribution of enriching gases and provides for the improved diffusion, mixing, and heating of these introduced enriching gases.